As well known to those skilled in the art, the carburators of the engines of automobiles are broadly classified by the type of venturi. There are fixed venturi type carburators and variable venturi type carburators such as SU carburator. Both of these types of carburators have their own advantages and disadvantages, and have been used depending on the performance desired.
Recently, regulations have been issued to limit the noxious exhaust emissions such as CO, HC, NO.sub.2 and so forth from automobile engines, in order to prevent environmental pollution.
Conversely, there is an increasing demand for carburators which can cope with the requirements for improved fuel consumption and safety.
The variable venturi type carburator such as SU type carburator has no shunting of main and slow fuel systems nor any connection system and, accordingly, has good transient response characteristics of the fuel supply to the change of flow rate of air-fuel mixture due to the opening and closing of the throttle valve. Therefore, the variable venturi type carburator is preferred for the exhaust emission control, as well as for improvement in fuel consumption and driveability. For this reason, it is becoming popular to install the variable venturi type carburator, which has heretofore been used only in automobiles designed for sport use or the like, on conventional automobiles. These variable venturi carburetors have, despite their mentioned advantages, problems. This invention is directed to the solutions of those problems.
In order to illustrate the problems of the conventional variable venturi type carburetor, reference is made to FIG. 1 which shows a typical conventional varibale venturi carburetor of the inclined type. Referring to FIG. 1, a conventional variable venturi carburetor is shown having a choke valve 1 which is disposed in an air horn 2 which is connected through a casing 6 to the bore 5 of a mixing chamber 4 having a throttle valve 3. The casing 6 has a piston guide 7 slideably receiving the reduced-diameter portion of a suction piston 8, while the increased-diameter portion of the suction piston 8 is adapted to slide in a suction chamber 9 provided in the casing 6. A piston rod 10 press-fitted to the suction piston 8 and centered on the latter makes a sliding contact with the guide rod 11 of the suction chamber 9 to thereby vary the cross-sectional area of the venturisection 12 in the bore 5.
A metering rod apparatus is connected to the piston 8 to move therewith to admit fuel from reservoir into venturi section 12 when throttle valve 3 is opened. The carburetor shown has the usual features such as a low speed idle jet and fuel metering valve.
Basically, the displacement or stroke of the suction piston 8 is determined by the vacuum generated at the venturi section 12 and the force of a compression spring 13 interposed between the inner end of the suction piston 8 and the inner other end of the suction chamber 9.
It is often the case that the suction piston 8 is caused to move back and forth in an oscillating manner, due to a pulsation of the intake vacuum during the low speed running of the engine, resulting in the undesirable vibration or pulsing of the suction piston 8.
Another problem occurs when the throttle valve 3 is opened abruptly causing the suction piston 8 to move in an impacting manner, resulting also in an undesirable vibration or an overshoot.
These undesirable vibrations and overshoots of the suction piston 8 in turn cause various problems such as an excessive leaning of the mixture, thus hindering the smooth operation of the engine and the performance of the vehicle.
One solution used to avoid these problems has been to fill the suction piston rod 10 with damper oil 14 which prevents the vibration of the suction piston 8 by the damping effect provided by the damper oil 14 as piston rod 10 slides within guide 11.
The oil is kept within hollow portion of rod 10 by a seal which is affixed by a rod and a plug to the casing 6. Thus, the oil acts as a compression spring.
The damper oil 14, however, leaks out and is depleted as time elapses. In order for this arrangement to function efficiently, it is necessary to replenish the oil periodically.
Just when to replenish the oil is unknown since the rate of depletion will vary depending on how and when the vehicle is driven and the overall dependability of the carburetor.
In addition, it is necessary that the viscosity of the damper oil 14 remain constant in order that the latter may perform the required damping function. If the damping oil 14 is too viscous, the speed of movement of the suction piston 8 is lowered to inhibit the response characteristic thereby making the air-fuel mixture excessively rich which in turn increases the noxious exhaust emissions and rate of fuel consumption. To the contrary the required damping effect cannot be obtained if the viscosity is too low which may cause the carburetor to provide an inadequate supply of fuel to the engine.
Further, the viscosity of the damper oil 14 changes depending on the temperature. Consequently, smooth engine operation is often impossible after a cold start, and during continous running at a high temperature for long durations. I.e., the carburetor which has been set for optimum operation in the cold winter season cannot operate adequately in the hot summer season.
Thus, the conventional varibale venturi type carburetor utilizing damper oil requires extremely maintenance and even if it is performed is too sensitive to temperature variances which normally occur within the year or within a given season of the year.
Various types of carburetors having improved oil dampers have been proposed as, for example, in Japanese Patent Laid-Open Publication No. 1724/1978, as a solution to the problems of oil damped carburetors.
Also, a mechanism has been proposed in which, as illustrated in FIG. 1 a suction hole 15 is provided in the suction piston 8, so as to impart a delay or time lag to the working of damper oil 14. This type of mechanism, however, is inefficient in that the clearance provided in the sliding fit between the guide rod 11 and the small diameter portion of the suction piston 8 is too small so that no breaking effect which would assist the damper oil is effected.
Various variable venturi type carburetors having no oil damper have also been proposed. These carburetors, however, have an extremely complicated construction and require troublesome maintenance and, therefore, are not suitable for practical use because of their high cost of production and maintenance.